Epitaxy is a process by which a single crystal, for example Silicon, is grown on or deposited on a single crystal substrate. Exemplary processes include chemical vapor deposition (CVD), wherein gas phase Silicon sources, such as silicon tetrachloride (SiCl4), trichlorosilane (SiHCl3), clichlorosilane (SiH2Cl2) and/or silane (SiH4) in a hydrogen carrier gas, are passed over a silicon substrate at a high temperature, e.g., about 700° C. to 1200° C., resulting in an epitaxial growth process.
Epitaxy is an important process in semiconductor manufacturing, and is often used to grow layers of pre-doped silicon on the polished planar surfaces of silicon wafers, before they are processed into semiconductor devices. Epitaxy is commonly used in the fabrication of power semiconductor devices, such as those used in computer power supplies, pacemakers, vending machine controllers, automobile computers, and the like.
FIG. 1A (conventional art) illustrates one well known deleterious side effect of epitaxial growth, known as “auto-doping.” Auto doping is a process by which dopants originating from the substrate 110 migrate into the epitaxial layer 120, deleteriously changing the doping profile of the epitaxial layers. It is appreciated that dopant migration may take a variety of paths from a substrate to an epitaxial layer, including, for example, liberation into the process gas(ses). In general, auto doping may lead to numerous adverse effects, including, for example, reduced breakdown voltage of the epitaxial layer. Additionally, the auto doping process is generally neither controlled nor predictable. Thus, auto doping leads to numerous detrimental effects.
A wafer may have an optional oxide seal 125 on the back side. Oxide seal 125 is intended to reduce auto-doping. However, oxide seal 125 may corrode and be subject to “pin-hole” defects during multiple cleaning processes between multiple epitaxial layer growth processes. When oxide seal 125 is subject to such corrosion, the oxide seal 125 fails to prevent auto doping.
An additional problem with epitaxial processes occurs when epitaxy undesirably grows on the back side of a wafer. FIG. 1B (conventional art) illustrates an irregular silicon “bump” or nodule 130 that has formed the back or opposite side of wafer 110/oxide seal 125 due to exposure to process gasses. Pin-hole defects of oxide seal 125 are prime locations for formations of such nodules, although such nodules may form at other locations, including in the absence of oxide seal 125. Such bumps are typically not uniform. For example, such inadvertent back-side epitaxial growth does not form a unified, smooth layer, but rather forms a plurality of irregular bumps. Such nodules present an uneven wafer backside, which may interfere with many subsequent semiconductor processing steps, as they prevent accurate alignment of the wafer in processing machinery. For example, as illustrated in FIG. 1B, the wafer 110 cannot lay flat due to nodule 130.
While there are many systems and methods to mitigate both auto doping and inadvertent back side epitaxial growth, including acceptance of the effects, such conventional art approaches are not acceptable in all circumstances. In addition, it is known to utilize multiple epitaxial layers. In such a circumstance, the accumulation of auto doping and/or back side nodules due to multiple epitaxial growth processes may overwhelm or otherwise find unsatisfactory conventional mitigation techniques.